Answer

Work Step by Step

a) The number of pages in this text can be counted. A result of counting is an exact number.
b) The number of days in the month of January is also a result of counting within the definitions of day and January. So there are no errors in measurement, which makes that it is an exact number.
c) The area of a city lot cannot be counted but has to be measured, so it is a measured quantity subject to uncertainty.
d) The distance between the centers of the atoms in a gold medal can be measured by chemists. So it is a measured quantity subject to uncertainty.